Strange Behavior of Typescript:
export type MyType = 0 | 1 | 2;
The above code snippet functions correctly. However, the following code snippet encounters an issue:
export const ONE = 1;
export const TWO = 2;
export const THREE = 3;
export type MyType = ONE | TWO | THREE;
By looking at the second instance, you're operating within typespace by utilizing a value (the declared constants) to define a type. To specify a type, you should use typeof ONE, TWO, or THREE. Conversely, in the initial example, 1, 2, and 3 are considered types on their own.
The issue at hand is that you have defined MyType as a type while referencing a variable within it. To properly match types, you should request the type of your value using the typeof operator in the following way:
export const ONE = 1;
export const TWO = 2;
export const THREE = 3;
export type MyType = typeof ONE | typeof TWO | typeof THREE;
To start, it is recommended to utilize the typeof operator as mentioned in a previous response.
Additionally, if I interpret your query correctly, you aim to construct a type that specifically encompasses these exact three provided values. In order to achieve this, you should employ the as const keywords.
Here is an illustration:
export const X = 1 as const;
export const Y = 2 as const;
export type CustomType = typeof X | typeof Y;
Furthermore, consider this practical example utilizing const variables:
export const state = {
ACTIVE: 1,
INACTIVE: 2
} as const;
export type StateType = typeof state[keyof typeof state];
export interface IEntity {
id: number;
status: StateType;
}
// This can be applied elsewhere in this manner:
const Z: IEntity = {
id: 1,
status: state.ACTIVE
};
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